1. Field of the Invention
The present invention relates to a solid state image pickup device, and more particularly to a solid state image pickup device for detecting focus (hereinafter, also referred to as AF) and a camera using the solid state image pickup device for detecting focus.
2. Related Background Art
An explanation will be made on a conventional solid state image pickup device used for a camera for detecting focus based on a phase difference detection system. According to the conventional solid state image pickup device, an object as a subject of focus detection is projected on a photosensor array on the solid state image pickup device to obtain an electrical signal output to which photoelectric conversion has been applied, in the form of an image. Photocarrier generated in the photosensor array is subjected to charge-voltage conversion to be stored in a capacitor for holding a signal. During accumulation processing, auto gain control (peak monitor control system) is generally adopted to monitor the maximum output value based on a contrast of the object that spreads over a wide area, to thereby control an accumulation time. The above is disclosed in Japanese Patent Application Laid-Open No. S61-045685.
After accumulation end, the obtained image signal is inputted to an analog to digital converter (ADC) to be subjected to arithmetic operation processing by a microcomputer. If a distance to the subject of focus detection is obtained through the arithmetic operation processing by the microcomputer, a motor is driven to control a lens for focusing. The image signal outputted from the solid image pickup device is subjected to signal processing so that an input dynamic range of from the ADC can be effectively used. The signal processing includes level shift processing and gain processing. According to the level shift processing, a specific signal voltage outputted from the photosensor array is adjusted to an arbitrary output reference voltage. According to the gain processing, a gain is applied to a level-shifted signal. In the level shift processing, it is common to adjust a signal voltage of an optical black (OB) pixel arranged in the photosensor array to an arbitrary output reference voltage. This is referred to as dark reference output system.
However, in a case where the peak monitor control and the dark reference output are used in combination to perform phase difference detecting as described above, it may not be possible to detect focus for a low-contrast object which produces a small output in amplitude. In view of this, a peak bottom reference output is used, in which a difference between a maximum output value and a minimum (bottom) output value is monitored to control an accumulation time while time the minimum output value is compared with an output reference voltage to determine the level shift amount. Japanese Patent Application Laid-Open No. 2003-107340 discloses the above.
FIGS. 10A and 10B show conventional examples in which the peak monitor control and the dark reference output are combined, and FIGS. 11A and 11B show another conventional examples in which the peak bottom monitor control and the peak bottom reference output are combined. Each example shows an input signal (a) to an amplifier circuit, and output signal (b) obtained by subjecting the input signal (a) to the level shift processing and the gain processing. In the case where the peak monitor control and the dark-reference output are combined, accumulation ends when the maximum value outputted from the photosensor array reaches a full accumulation level. After the accumulation end, the signal voltage of the OB pixel is level-shifted to coincide with the output reference voltage, and applied with a gain. Also, in the case where the peak bottom monitor control and the peak bottom reference output are combined, accumulation ends when a voltage difference between the maximum output value and the minimum output value of the photosensor array reaches the full accumulation level. After the full accumulation, the minimum output value of the photosensor array is level-shifted to coincide with the output reference voltage, and applied with a gain.
However, according to the dark reference output described above, it may not be possible to detect focus for a low-contrast object which produces a small output in amplitude. According to the peak bottom reference output described above, a large output in amplitude can be obtained from a low-contrast object, but a bottom detecting circuit is necessary, which increases a chip area and power consumption.